Here are my ideas. The first thing is you should do a self-assessment on how you are caring for your horse and honestly answer these questions: is his stall getting cleaned properly? Any feces left around more than twelve hours is something I would prefer not to have with any horse. It makes the barn smell. Ditto for urine. You could always do a mini muck if much of the straw or bedding is still good. Does he get enough food and water? Does he get enough time out to keep him from getting bored?
Now, someone mentioned supplementing, I think that’s okay, but I’d really like my vet to do a physical and blood workup of a horse that has suddenly started to coprophage. If there is a physical ailment, no amount of supplementing will help until that’s been cured!